roberts



I. L. ROBERTS PROPBLLBR June 29 ,1926. Re; 16,370

Original Filed c 15,1918 2 Sheets-Sheet 1 June 29 1926.

- l. 1.. ROBERTS PROPELLERI Original Filed 11- 15, 1918 2 Sheets-Sheet 2 fiw K as, @WWMVB minimum of obstruction to the passage of the water through the central portion of the propeller.

The concave arms 5, 6, 7, 8 terminate in blades 1, 2, 3, 4, whose working surfaces are circumferentially extending portions of helicoids, and both the inner and outer edges of these blades extend along helices havingfthe same'pitch.

A ropeller made in the form shown in the rawlngs and described above yields a much higher elliciency' than any other known tome. The arms yield a proportional return of thrust for the power aplied to, them as do the terminal blades and have greater resistance to slip than plain arms or a helix. Theirform and the eject- I ing of the water to the rear in lines directly opposed to the progress of the bca't, causes an equal straight thrust at all times.

The rear termination of the concavity of the arms pointing directly rearward produces a greater efiiciency than can be obtained if the water is ejected at any angle to the line traversed by the boat in its progress. The convexity of the opposite side to the concavity produces a suction when the arm is rotating and this suction is in front of the arms and in the direction the boat is traveling and materially adds to the thrust. The curve of said arm in an arc of a circle minimizes the resistance to the passage or rotary movement thereof through the water. The concavity further greatly strengthens the arms and necessitates less metal in construction. The angular projections at 10, 11, 12, '13 rearward onthe arms shown, add strength to the arms and being tapered to a sharp angle rearwardly tend to prevent cavitating at high speeds.

The blades in area greatly exceed their respective arms or supports. The objects of making said vanes of such area is twofold. The propelling force of any proeller is greatest at its greatest distance groin the center of the hub, area for area thereof, considered; also the quantity or area of cross-section of water out through in revolving is greatest and as the water is the abutment against which the propeller ts its thrust it is important to secure as arge an abutment as ossible.

.No exact rule can ere be given for the percentage of area of these blades to the cylinder of water out in the rotation of the entire propeller; but tests show that the aggregate projected area of all the blades gives highly eflicient results when this area is about forty to sixty per cent of the projected area of the propeller. This percentage may however be varied for conditions of use. In the case of towing, where the speed of the boat is slow and a strong pull is required, blades of larger area may be used, while on high speed boats blades of smaller area may be used to advantage. 'llhe greater the area the lesswill be the s 1p.

structed as above described is due to several factors, which include (1) the form and ar- The high etli'ciency of a propeller con rangement of the arms, whereby there is afforded easy passage ofthe water through the central portion of the propeller, as the boat advances, with the lowest resistance to or disturbance of this central column of water; (2) the fact that the arms though relatively small yet exert considerable thrust operative, because of the form of the concavity, to force the water straight back; (3)-the relatively large area of blade surface located at the greatest possible dis tance from the axis, whereby slip is lessened to a high degree; and (4) the exact curvature of the edges of the blades to the curve of the spiral through which they travel in rotating. v

This large area of blade and arm surface working effectively against the water, as an abutment, further obviates, to a large degree, the necessity for nice adjustment of the pitch of the propeller blades to the lines or speed of the particular vessel on which the propeller is to be used, and'the efficiency of the propeller is maintained withina wide range of variation of speed of travel of the boat or of variation in the rate of rotation of .the propeller. e

It will be noted, further, that the major portion of the area of the 1propeller is-located at its outer or perip eralpart and preferably beyond an imaginary cylindrical surface having a radius of about five-eighths that ofthe propeller as a whole. The blades lie approximately between two limiting cylindrical surfaces or in cylinders co-axial with the propeller.

The diameter of the propeller, as herein shown and described, is 16 inches and the pitch is 22 inches, which may however be made greater or less according to the speed at which the boat is designed to go. As the rearward concave warped surfaces of the propeller arms have the same pitch as the working blade surfaces and these surfaces merge into each other, the propeller as a whole closely approaches the ideal, condition of a screw working into the water as into a solid nut. This ideal condition is more closely approximated'because of the particular construction of the arm portions. which permits free flow of water past them. The form of the pro ller shown in Fi 1 and 2 may be modi ed to that shown in Figs. 7 and 8, which 0 rates in the same manner, but has the vantage that there is less liability of the blades catching in i Jkarm-portions 5', 6", 7, 8 are of the same construction and arrangement as the arms to or weeds.

.Ethe form shown in Figs. 7 and 8, the

5 6, 7, and 8; but the blades 1, 2, 3, 4'

- variations from this form may are extended, as shown, to one side only of without departure from the spirit of my invention.

' I ckiim: l f 1. prope ler com risin a plurality o circumferentially axially extending helicoidal blades and arms lying in substantiall the same radial plane connecting the bla es to the shaft, said arms havin concaved propelling surfaces whose cor s are set on pitch lines co-ordinating with the pitch lines of the helicoidal blades.

2. Propeller according to claim 1 in which the arms are axially curved, their convex edges pointing in the direction of rotation.

3. Propeller according to claim 1 in which the concave propelling surface of each arm is defined by progressively varying radii of curvature, the radius being shortest adjacent the hub and longest adjacent the line of union with the blade.

In witness whereof I have subscribed my name to this specification.

ISAIAH L. ROBERTS. 

